Photoacoustic spectroscopy apparatus and method

ABSTRACT

The invention encompasses photoacoustic apparatuses and photoacoustic spectrometry methods. The invention also encompasses sample cells for photoacoustic spectrometry, and sample cell/transducer constructions. In one aspect, the invention encompasses a photoacoustic spectroscopy apparatus, comprising: a) a sample reservoir and an acoustically-stimulable transducer acoustically coupled with the sample reservoir, the transducer comprising a detector surface having a substantially planar portion; and b) a beam of light configured to be directed through the sample at an angle oblique relative to the substantially planar portion of the detector surface to generate sound waves in the sample. In another aspect, the invention encompasses a photoacoustic spectroscopy sample cell, comprising: a) a first block of material having opposing front and back surfaces, the front surface comprising a substantially planar portion configured to be against a sample and the back surface comprising a substantially planar portion configured to be joined to a transducer, the back surface being parallel to the front surface; and b) a pair of opposing side surfaces joined to opposite ends of the front and back surfaces, one of the opposing side surfaces being configured for passage of light therethrough and extending at a first oblique angle relative to a plane containing the substantially planar portion of the front surface.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional of application Ser. No.09/105,781 filed Jun. 26, 1998, now U.S. Pat. No. 6,236,455.

TECHNICAL FIELD

The invention pertains to photoacoustic spectroscopy, including methodsof photoacoustic spectroscopy and photoacoustic spectroscopyapparatuses.

BACKGROUND OF THE INVENTION

Photoacoustic spectroscopy is an analytical method that involvesstimulating a sample by light and subsequently detecting sound wavesemanating from the sample. Typically, only a narrow range of wavelengthsof light are introduced into a sample. Such narrow range of wavelengthsof light can be formed by, for example, a laser. Utilization of only anarrow range of wavelengths can enable preselected molecular transitionsto be selectively stimulated and studied.

A photoacoustic signal can occur as follows. First, light stimulates amolecule within a sample. Such stimulation can include, for example,absorption of the light by the molecule to change an energy state of themolecule. Second, an excited state structure of the stimulated moleculerearranges. During such rearrangement, heat, light, volume changes andother forms of energy can dissipate into an environment surrounding themolecule. Such forms of energy cause expansion or contraction ofmaterials within the environment. As the materials expand, sound wavesare generated. Accordingly, an acoustic detector mounted in acousticcommunication with the environment can detect changes occurring as aresult of the light stimulation of the absorbing molecule.

An exemplary prior art apparatus 10 for photoacoustic spectroscopy isshown in FIG. 1. Apparatus 10 comprises a light source 12 configured toemit a beam of radiation into a sample holder 14. Light source 12 cancomprise, for example, a laser. Filters (not shown) can be providedbetween light source 12 and sample holder 14 for attenuating the lightprior to its impacting sample holder 14.

Sample holder 14 comprises a sample cell 18 containing a sample 16.Sample cell 18 can comprise a number of materials known to persons ofordinary skill in the art, and preferably comprises a materialsubstantially transparent to the wavelengths of light emanating fromlight source 12. Preferred materials of sample cell 18 will accordinglyvary depending on the wavelengths of light utilized in the spectroscopicapparatus. If the wavelengths of light are, for example, in the range ofultraviolet through visible, sample cell 18 can preferably comprisequartz.

Sample 16 comprises a material that substantially fills sample cell 18.Such material can be, for example, a fluid such as a liquid or a gas.Sample 16 can, for example, comprise a liquid solution wherein themolecular vibrations that are to be studied are associated withmolecules dissolved in the liquid.

Apparatus 10 further comprises an acoustic detector 20 mounted to samplecell 18 and in acoustic communication with sample 16. Acoustic detector20 can comprise a transducer, such as, for example, a microphone and canbe mounted such that a fluid (for example, a grease) is provided betweena surface of detector 20 and sample cell 18. Detector 20 is typicallyremovably mounted to sample cell 18 by, for example, a clamp. Acousticdetector 20 is in electrical communication with an output device 22.Device 22 can be configured to display information obtained fromdetector 20, and can be further configured to process such information.Output device 22 can comprise, for example, an oscilloscope or acomputer.

In operation, a beam of light is generated by source 12 and passedthrough sample cell 18 to stimulate molecular excitation within sample16. Non-radioactive decay or molecular rearrangements cause expansionsand/or contractions of a material within sample 16 to generate acousticwaves passing from sample 16 through sample cell 18 and to acousticdetector 20. Acoustic detector 20 then detects the acoustic waves andpasses signals corresponding to, for example, amplitudes and frequenciesof the acoustic waves to output device 22. Output device 22 can beconfigured to convert information obtained from detector 20 to, forexample, a graphical display.

A difficulty in utilizing apparatus 10 is that acoustic waves emanatingsimultaneously within sample 16 do not reach detector 20 at the sametime. As shown in FIG. 2, light from source 12 typically has a generalshape of a cylinder 24 as it passes through sample cell 18. Individualacoustic waves emanating from cylinder 24 (shown as dashed lines 26)also have cylindrical shapes. All portions of an individual acousticwave 26 are generated simultaneously within sample 16, and shouldtherefore desirably simultaneously impact detector 20. However, asacoustic detector 20 has a flat detection surface, an individualacoustic wave 26 will impact acoustic detector 20 at a later time at anedge of the detection surface relative to a center of the detectionsurface. Thus, there is a spread of a time interval during which anindividual acoustic if wave impacts detector 20, rather than the desiredsimultaneous detection event. It is desirable to reduce the timeinterval during which an. individual acoustic wave is detected toenhance sensitivity.

One approach that has been utilized for reducing such time interval isto utilize a detector 20 having a curved detection surface approximatelycomplementary to the curved cylindrical shapes of acoustic waves 26.However, as such detectors can be difficult to make the approach has hadlimited success. Another approach is to use a slit to provide a planaracoustic wave.

Another approach that has been utilized for reducing a time intervalduring which an individual acoustic wave is detected is exemplified by aphotoacoustic apparatus 10 b shown in FIG. 3. In referring to theapparatus of FIG. 3, similar numbering to that utilized above indescribing apparatus 10 of FIG. 1 will be used, with differencesindicated by the suffix “b” or by different numerals. The primarydifference between apparatus 10 b and apparatus 10 of FIG. 1, is that inapparatus 10 b transducer 20 is mounted directly in front of the beam oflight emanating from light source 12. Accordingly, apparatus 10 bcomprises a sample cell 14 b slightly modified from the sample cell 14of apparatus 10 (FIG. 1). As long as transducer 20 has a detector facethat is smaller in cross-sectional area than an area of the light beamemanating from source 12, individual waves generated by the light beamwill reach the face at approximately the same time across an entiresurface of such face. Accordingly, apparatus 10 b can eliminate theabove-discussed problem of individual acoustic waves reaching anacoustic detector face at a spread of time intervals across a surface ofthe face. A difficulty associated with apparatus 10 b is that the lightemanating from source 12 shines directly into a detector face oftransducer 20 and can adversely heat such face. Accordingly, a shield 26is typically provided along an internal sidewall of sample cell 18 b toblock radiation emanating from light source 12 from reaching a detectorface of transducer 20. Shield 26 is typically a thin film, and such thinfilms are typically only suitable for very narrow ranges of light (about20 nanometers on average). Accordingly, a band of light entering sampleholder 18 b must typically be kept to a very narrow wavelength range toavoid having light pass through film 26 and into transducer 20.

As the above discussion indicates, the apparatuses 10 and 10 b of FIGS.1 and 3, respectively, both have advantages and disadvantages.Specifically, the apparatus 10 of FIG. 1 can enable relatively largebands of light to be utilized for photoacoustic spectroscopyexperiments, but has slow response times and significantly lowersensitivity due to large time intervals wherein individual acousticwaves impact different regions of an acoustic detector surface. Incontrast, apparatus 10 b can have rapid response times to acoustic wavesgenerated within sample 16, but is generally only useful for relativelynarrow ranges of light. It would be desirable to develop alternativephotoacoustic detector systems which could accomplish the advantages ofboth apparatus 10 of FIG. 1 and apparatus 10 b of FIG. 3.

In another aspect of the prior art, it is recognized that light can beeither refracted or reflected by a material, depending on an angle iswith which the light impacts a surface of the material. Such isillustrated with respect to a material 50 in FIG. 4. Material 50comprises an upper surface 52. Upper surface 52 is substantially planar.An axis “X” extends normal (i.e., perpendicular) to planar surface 52. Acritical angle θ is defined as an angle relative to normal axis “X”wherein a beam of light impacting surface 52 passes from predominantlyreflecting from surface 52 to predominantly refracting within surface52. A critical angle is determined by the relative refractive indices ofmaterials joining at a surface. Specifically, if light passes from afirst material having a larger refractive to a second material with alesser refractive index, a critical angle can be defined relative to anaxis normal to a surface where the two materials meet. In the example ofFIG. 4, such surface corresponds to surface 52. If light impacts surface52 at an angle greater than angle θ, the light will predominantlyreflect from surface 52. Also, if light impacts surface 52 at an angleless than angle θ, the light will predominantly pass into material 50and refract within material 50. A critical angle θ for particularmaterials can be calculated from application of Snell's law and therelative amount of refraction and reflection can be determined. For aquartz/air interface a critical angle θ is about 40.4°, and for aquartz/water interface a critical angle θ is about 59.7°.

FIG. 4 also illustrates that a beam of light 55 can be directed intomaterial 50 at an appropriate angle such that the light reflects fromsurfaces of material 50 to be contained internally of material 50. Suchreflections are referred to as internal reflections. It is known thatsome of the light will actually extend slightly outward of a surface ofmaterial 50 (such as surface 52) as the light reflects internally fromthe surface. Such is illustrated by curved lines 57 in FIG. 4. Althoughthe light extends slightly outward of the surfaces of material 50 as itis reflected within material 50, the light continues along the generalpath illustrated by beam 55. Accordingly, if material 50 is providedadjacent a sample, a light beam 55 can be provided to be internallyreflective within material 50 and yet to stimulate molecules within thesample. Such use of internal reflections for stimulating moleculeswithin a sample can be advantageous in situations wherein a sample isgenerally not transparent to a light source, such as, for example, whenthe sample is relatively turbid or optically dense. The amount by whichlight waves penetrate into a sample can be adjusted by changing awavelength of the light, or by changing an angle at which the lightinternally reflects from surfaces of material 50.

In yet another aspect of the prior art, it is recognized that a sample'sabsorbance of light is directly proportional to a path length of lightthrough the sample, and to a concentration of an absorbing specieswithin the sample. Such relationship can be represented by the formulaA=abc, wherein A is absorbance, a is a proportionality constant calledabsorptivity, b is a pathlength of light through the sample, and c is aconcentration of absorbing species within the sample. Such relationshipis referred to as Beer's Law. The Beer's Law relationship indicates thatan amount of light absorbed is proportional to a concentration of anabsorbing species. Another way of describing absorbance is as Log P₀/P,wherein P₀ refers to the initial power of a light beam impacting asample and P refers to the power of the beam exiting the sample. Mostspectroscopic methods can detect and quantitate absorbing species onlywithin a very narrow range of absorbance, such as, for example, a rangeof from about 0.05 to about 1.0. Accordingly, samples must be eitherdiluted or concentrated to bring an absorbance of the sample within theappropriate range for the spectroscopic measurements. For samples thatare extremely dilute, such as minor contaminants in sea water, it can bedifficult and time consuming to adequately concentrate the samples forspectroscopic measurements. Accordingly, it would be desirable todevelop spectroscopic methods that could be utilized over a wide rangeabsorbance.

In contrast to spectroscopy methods which measure absorbance as LogP₀/P, photoacoustic spectroscopy measures only P. This can provideenhanced sensitivity relative to other forms of spectroscopy in that itdoes not involve measuring a small signal “P” in the presence of a largebackground “P₀” Also, an amplitude of a photoacoustic signal is believedto depend inversely on a volume of an excitation source (i.e., P/V₀). Inother words, Photoacoustic Theory predicts that an amplitude of aphotoacoustic signal is proportional to an energy/volume ratio, whereinthe energy is the energy generated by a measured transition and thevolume is the volume of a sample. Photoacoustic spectroscopy can thus beadvantageous over other forms of spectroscopy.

SUMMARY OF THE INVENTION

In one aspect, the invention encompasses a spectroscopy apparatusconfigured to enable direct measurement of absorbance across an entiretyof the range of from about 0.0001 absorbance units per centimeter toabout 10,000 absorbance units per centimeter.

In another aspect, the invention encompasses a photoacousticspectroscopy sample cell. The sample cell includes a first block ofmaterial. The first block of material has opposing front and backsurfaces. The front surface comprises a substantially planar portionconfigured to be against a sample. The back surface comprises asubstantially planar portion configured to be joined to a transducer.The back surface is substantially parallel to the front surface. Thefirst block of material also has a pair of opposing side surfaces joinedto opposite ends of the front and back surfaces. The opposing sidesurfaces are a first opposing side surface and a second opposing sidesurface The first opposing side surface is configured for passage oflight therethrough and extends at a first oblique angle relative to aplane containing the substantially planar portion of the front surface.The second opposing side surface extends at a second oblique anglerelative to the plane containing the substantially planar portion of thefront surface.

In yet another aspect, the invention encompasses a method ofphotoacoustic spectroscopy. A sample is provided and anacoustically-stimulable transducer is provided acoustically coupled withthe sample. The transducer comprises a detector surface having asubstantially planar portion. A first beam of light is directed throughthe sample at an oblique angle relative to the substantially planarportion of the detector surface. The first beam of light generates soundwaves in the sample. The sound waves are detected with the transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a schematic, diagrammatic view of a first prior artphotoacoustic spectroscopy apparatus.

FIG. 2 is a view along the line 2—2 of FIG. 1.

FIG. 3 is a diagrammatic, schematic view of a second prior artphotoacoustic spectroscopy apparatus.

FIG. 4 is a cross-sectional sideview of a prior art materialillustrating various relationships between angles and light wavesimpacting the material.

FIG. 5 is a diagrammatic, cross-sectional view of a photoacousticspectroscopy sample cell of the present invention.

FIG. 6 is a diagrammatic view along line 6—6 of FIG. 5.

FIG. 7 is a second diagrammatic, cross-sectional view of a photoacousticspectroscopy sample cell of the present invention.

FIG. 8 is a diagrammatic top view of a photoacoustic spectroscopy samplecell holder apparatus of the present.

FIG. 9 is a diagrammatic cross-sectional sideview of the photoacousticspectroscopy sample cell holder of FIG. 8.

FIG. 10 is a schematic diagram of a photoacoustic spectroscopy apparatusof the present invention.

FIG. 11 is a diagrammatic, cross-sectional sideview of an alternativeembodiment photoacoustic spectroscopy sample cell of the presentinvention.

FIG. 12 is a diagrammatic, cross-sectional sideview of anotheralternative embodiment photoacoustic spectroscopy sample cell of thepresent invention.

FIG. 13 a diagrammatic, cross-sectional sideview of yet anotheralternative embodiment photoacoustic spectroscopy sample cell of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIGS. 5 and 6 illustrate a photoacoustic sample cell 100 encompassed bythe present invention. Sample cell 100 comprises a first block ofmaterial 102 and a second block of material 104. Blocks 102 and 104 cancomprise a same material, or can comprise different materials from oneanother. An exemplary material for blocks 102 and 104 is quartz. Blocks102 and 104 are separated from one another by a shim. 106. Shim 106 cancomprise, for example, at least one of a flexible gasket material (suchas, for example, rubber or plastic), or a metallic material. Inpreferred embodiments, shim 106 will comprise an annular shape. In theembodiment shown, block 104 has a rectangular shape and shim 106 is anoval ring. In other embodiments (which are not shown), material 104 canhave other shapes, such as, for example, square, oval, or circular, andshim 106 can have other annular shapes corresponding to circular rings,square-shaped rings, or rectangular-shaped rings, for example.

Block 102 comprises front and back surfaces 110 and 112, respectively,and opposing side surfaces 114 and 116. Front and back surfaces 110 and112 are preferably substantially parallel to one another. Opposing sidesurfaces 114 and 116 are joined to opposite ends of front and backsurfaces 110 and 112. Opposing side surface 114 can be referred to as afirst opposing side surface, and opposing side surface 116 can bereferred to as a second opposing side surface.

Cell 100 further comprises a sample reservoir 120 defined by shim 106,and blocks 102 and 104. Sample reservoir 120 is configured to hold amaterial, such as, for example, a liquid or gas that is to bephotoacoustically analyzed. Blocks 102 and 104 define walls of reservoir120.

Front surface 110 comprises a substantially planar portion configured tobe against a material contained within reservoir 120. The term“substantially” in reference to the substantially planar portion ofsurface 110 indicates that a so-called “planar” portion of surface 110can have structural features which cause it to vary from perfectplanarity, and yet still be sufficiently planar for purposes of thepresent invention. Such structural features can be introduced as, forexample, minor manufacturing defects. In preferred embodiments, thesubstantially planar portion of surface 110 extends entirely acrosssample reservoir 120.

Second block 104 comprises a front surface 130, a back surface 132, afirst opposing side surface 134 and a second opposing side surface 136.Front and back surfaces 130 and 132 are preferably substantiallyparallel to one another. Front surface 130 of second block 104 comprisesa substantially planar portion configured to be against a materialcontained within reservoir 120. In the shown embodiment, block 104 issubstantially identical to block 102. The term “substantially” indicatesthat block 104 can vary from block 102 by the presence of minormanufacturing defects, and yet still be identical for purposes of thepresent invention. Blocks 102 and 104 are preferably identical in shapewhen the blocks comprise identical materials. In embodiments in whichblocks 102 and 104 comprise different materials, it can be preferablefor blocks 102 and 104 to have different dimensions from one another.

In operation, a light beam 150 is passed through first surface 114 tosample reservoir 120. Light beam 150 preferably enters surface 114 at anangle perpendicular (normal) to surface 114 to minimize reflection ofbeam 150 from surface 114. Surface 114 extends obliquely at an angle αrelative to a plane containing the substantially planar portion of frontsurface 110 that is against sample reservoir 120. In the shownembodiment, an entirety of surface 110 is within such plane.Accordingly, oblique angle α is shown at a corner between surface 114and surface 110. For purposes of interpreting this disclosure and theclaims that follow, an oblique angle is defined as an angle that isneither 0° nor 90°.

An axis “Q” extends normal to surface 110. Light beam 150 strikessurface 110 at an angle β relative to axis “Q”. Angle β is determined bythe angle α. Specifically, angle β equals angle α. Accordingly, angle αcan be configured to provide beam 150 at less than, greater than, orequal to a critical angle of the material of block 102 relative tosurface 110. If angle β is less than such critical angle, a predominateportion of beam 150 will penetrate sample reservoir 120 along a pathsuch as that illustrated by dashed line 152. If angle β is greater thana critical angle of material 102 at surface 110, a predominate portionof light beam 150 will reflect from surface 110 along a path such asthat illustrated by dashed line 154. Accordingly, block 102 can beconstructed for either internal reflection of light beam 150 withinblock 102, or refraction of light beam 150 through reservoir 120 Ofcourse, the above-discussed equality of angles α and β only holds truein situations wherein α is from 0° to 90°. Preferably, angle α isgreater than 0° and less than 90°, and more preferably is greater than20° and less than 70°.

Although it can be preferred to have angles α and β equal to one anotherwhen α is between 0° and 90°, it can also be preferred that angles α andβ not equal to one another. For instance, it can be preferred to changean orientation of sample cell 100 relative to a beam of light (either bymoving sample cell 100 or by moving the beam) to vary the angle β atwhich the light impacts surface 110. Such can be preferred, for example,in circumstances in which it is desired to perform some measurements ona sample under conditions in which light travels along a predominatelyrefractive path (such as path 154) and other measurements underconditions in which light travels along a predominately reflective path(such as path 152). As angle α is generally fixed, angle β will notequal angle α at both the refractive conditions and the reflectiveconditions.

Second opposing side surface 116 forms an oblique angle γ relative tothe substantially planar portion of surface 110 configured to be againsta material within sample reservoir 120. Also, surface 130 of secondblock 104 comprises a substantially planar portion configured to beagainst a sample in reservoir 120. First and second opposing sidesurfaces 134 and 136 of second block 104 form oblique angles δ and ε,respectively, relative to such planar portion of surface 130. Obliqueangles α, γ, δ and ε are preferably substantially identical inembodiments in which blocks 102 and 104 consist of identical materials.Specifically, in such embodiments it can be desirable for light beam 150to enter first block 102 substantially perpendicular to surface 114 andto exit second block 104 at an angle substantially perpendicular tosurface 136. If blocks 102 and 104 consist of identical materials, suchcan be accomplished by having oblique angles α and ε be substantiallyidentical to one another. If blocks 102 and 104 consist of differentmaterials, it can be desirable to vary oblique angle ε relative tooblique angle α such that light exits block 104 in a directionsubstantially perpendicular to surface 136.

It can be advantageous to have oblique angles δ and γ identical to oneanother in experiments in which at least two beams of light are to bepassed through a sample. In such experiments, a first beam of light canbe passed along the path of beam 150, and a second beam of light can bepassed along a path which enters at surface 134 and exits 16 at surface116. Accordingly, the paths of the two beams of light will intersectsubstantially perpendicularly to one another within sample reservoir120. The beams of light can be passed through reservoir 120simultaneously with one another. Alternatively, the beams of light canbe passed in rapid succession such that the first beam of light excitesmolecules to an initial state, and the second beam of light eitherfurther excites the molecules to another state, or provides the excitedmolecules with a path of relaxation. The beams of light can compriseeither identical wavelengths, or different wavelengths from one another.Also, oblique angles α and δ can be configured such that one beam oflight predominately refracts through reservoir 120, and another beam oflight predominately internally reflects from one of surfaces 130 or 110.Further, the direction of one of the beams of light can be reversedrelative to a direction of the other beam of light. Additionally, it isnoted that the beams of light can comprise multiple wavelengths, some ofwhich predominately refract through reservoir 120 and others of whichpredominately reflect from one or both of surfaces 110 and 130.

An advantage of utilizing refraction and reflection in a commonphotoacoustic spectroscopy device is that such can enable the device tobe utilized for detecting and quantitating characteristics of samplesover a wide range of absorbances. Specifically, refraction-basedphotoacoustic methods can enable detection and quantitation of lowconcentrations of Is detectable components in samples (for example,detection can occur to at least as low as about 0.0001 absorbance unitsper centimeter), and internal-reflection-based photoacoustic methods canenable detection and quantitation of high concentrations of detectablecomponents in samples (for example, detection can occur to at least ashigh as about 10,000 absorbance units per centimeter). Thus, embodimentsof the present invention can enable detection and quantitation of samplecomponents having absorbances of from about 0.0001 to about 10,000. Thepresent invention can thus provide an expanded useful absorbance rangerelative to other for ms of spectroscopy. Such expanded range can enablemethods of the present invention to be utilized for directly analyzingsamples that would need to be significantly diluted or concentrated forother forms of spectroscopy. Experiments have been conducted to detectand quantitate Cr(VI) absorbance of 372 nanometer light at variousconcentrations of Cr(VI). Such experiments confirm that an apparatus ofthe present invention can be utilized to directly detect and quantitatea concentration of a sample component having an absorbance of from about0.0001 absorbance units per centimeter to about 10,000 absorbance unitsper centimeter. For purposes of interpreting this disclosure and theclaims that follow “direct” detection and quantitation of an absorbingspecies in a sample is defined to mean spectroscopic detection andquantitation that occurs without modifying a concentration (absorbance)of the absorbing species (by, for example, is concentration or dilution)prior to the detection and quantitation. In other words, “direct”detection refers to in situ, real time analysis.

It is noted that measurements of the detection limits of a sample cellof the present invention (such as cell 100 of FIG. 5) in both arefraction mode and a reflection mode indicate that operation of thecell cannot be explained entirely by either Beer's Law or PhotoacousticTheory. Specifically, the refraction mode has a detection limit abouttwenty-times larger, relative to the reflection mode, than that whichwould be predicted by Photoacoustic Theory alone, and yet the signal isseveral times smaller than that which would be predicted by Beer's Lawalone. It is to be understood that the scope of this disclosure is to bedetermined by the claims that follow, and is not to be limited to anyparticular mechanism except to the extent that such is expresslyclaimed.

A transducer 170 is coupled to back surface 112 of block 102. Transducer170 is preferably an acoustic microphone acoustically coupled with asample in reservoir 120 through block 102. In the shown embodiment, onlyone transducer is provided. However, the invention encompasses otherembodiments (not shown) wherein a second transducer can be provided at,for example, surface 132 of second block 104. An electrical interconnect172 extends from transducer 170 to electrically couple transducer 170with circuitry (not shown) for either processing or displaying signalsgenerated by transducer 170.

A method of operation of sample cell 100 is described with is referenceto FIG. 7. A sample 190 is provided within reservoir 120 and a beam oflight 180 is passed through surface 134 of block 104, refracted throughsample 190, and then exits from sample cell 100 through surface 116 ofblock 102. Sample 190 can comprise, for example, a fluid. Alternatively,sample 190 can comprise a solid, such as, for example, a powder or ablock having a smooth surface to align with an interior surface of block102. As another example, sample 190 can comprise an interface of twophases, such as a liquid/solid interface.

The light stimulates molecules within sample 190 to generate acousticwaves 185 which pass through block 102 and are detected by transducer170. It is noted that since the speed of light is several orders ofmagnitude greater than the speed of sound, light beam 180 effectivelyfills an entire thickness of reservoir 120 instantaneously prior toemanation of acoustic waves from sample 190. Acoustic waves 185 are thusgenerated to align parallel with surface 110 of block 102 (and travel ina direction perpendicular to surface 110).

Transducer 170 comprises a detector face 174 against surface 112 ofblock 102. In preferred embodiments, detector face 174 is substantiallyparallel with surface 110. Accordingly, detector face 174 issubstantially parallel to the alignment of waves 185. Detector face 174preferably comprises a surface area less than a surface area of acousticwaves 185. Specifically, detector face 174 preferably comprises asurface area less than an area of sample 190 stimulated by light beam180. In such preferred embodiments, an entirety of detector face 174 canbe stimulated simultaneously by individual acoustic waves 185.

An exemplary apparatus 200 for holding sample cell 100 is shown in FIGS.8 and 9. FIG. 8 shows a top view of such apparatus, and FIG. 9 shows asideview. Apparatus 200 comprises a support structure 202 with a flatbase 204. A post 206 extends into support structure 202 and can beconfigured to move within structure 202 for height adjustment of samplecell 100.

Apparatus 200 further comprises a holding box 208 supported on post 206.Holding box 208 comprises sidewalls 210 and 212 and a base 214.Sidewalls 210 and 212, as well as base 214, can be formed of, forexample, stainless steel. A tension adjustment pin 216 is threadedlyengaged within sidewall 210 and is coupled to transducer 170 with acushioned end 218. Cushioned end 218 can comprise, for example, a rubbermaterial joined to pin 216. Pin 216 can be screwed into sidewall 210 toprovide tension against sample cell 100 for retaining sample cell 100within box 208. The sample cell/transducer assembly shown in FIGS. 8 and9 comprises a second transducer 190 joined to second block 104 of samplecell 100. Transducers 170 and 190 are electrically coupled to processingand/or output circuitry through electrical interconnects 172 and 192,respectively. In the shown embodiment; block 104 comprises an inlet hole220 and an outlet hole 230 for continuously flowing a sample intoreservoir 120 (FIG. 5). Holes 220 and 230 are connected to ports 240 and250, respectively. It is preferred to have outlet hole 230 above inlethole 220 so that if air is introduced into reservoir 120 (FIG. 5) itwill be readily expelled from sample cell 100. The embodiment shown inFIGS. 8 and 9 can be advantageous for continuously monitoring samples.Such continuous monitoring can be desired, for example, in environmentalapplications wherein samples are to be monitored for pollution or othercontaminants, and in applications wherein samples are to be monitoredfor time-dependent changes.

FIG. 10 schematically illustrates a photoacoustic spectroscopicinstrument 300 configured for incorporating a sample cell 100 of thepresent invention. Instrument 300 comprises a laser 310 configured toemit a beam of radiation. Such beam of radiation is directed by a wedge320 through a filter wheel 330, a beam splitter 340, and an iris 350,and into sample cell 100. Wedge 320, filter wheel 330 and iris 350 canbe provided to attenuate the beam of radiation. Radiation thatpenetrates wedge 320 is directed to a beam stop 410 which blocks theradiation from entering an environment proximate apparatus 300. Beamsplitter 340 splits light from laser 310 into a first beam whichpenetrates sample cell 100, and a second beam which enters an energymeter 400. Energy meter 400 is coupled to a processor 380 and outputs asignal to processor 380 indicating that a laser pulse has occurred. Suchsignal can be utilized to trigger data acquisition by processor 380.

The beam passing through sample cell 100 impacts a photodiode 360configured to detect an intensity of the beam. Photodiode 360 is coupledto an output device 370 such as, for example, a digital oscilloscope,and to processor 380. Processor 380 can be configured to, for example,store information obtained from photodiode 360, or to graphically outputsuch information in the form of, for example, a graph of intensityrelative to time.

The beam from laser 310 generates an acoustic signal within sample cell100 that is detected by a transducer 170. A signal from transducer 170is passed to an amplifier 390. Amplifier 390 outputs a signal to outputdevice 370 and processor 380. Processor 380 can then, for example, storethe signal, or process the signal to, for example, output a graph ofacoustic signal relative to time.

The above-described embodiments are sample cells in which a samplereservoir is contained between two blocks of material. It is to beunderstood, however, that the invention also encompasses embodiments inwhich a sample reservoir is against a surface of a block, regardless ofwhether a second block is provided against a sample reservoir. Forinstance, FIG. 11 illustrates an embodiment of the invention in which asample cell comprising a single block of material 400 is in contact witha fluid sample 440. Fluid 440 is contained within a vessel 412. Block400 comprises a surface 418 in physical contact with fluid 440. Atransducer 414 is mounted to block 400 on a surface 416 parallel tosurface 418. A beam of light 420 is directed into sample cell 400 at anangle which reflects from surface 418. During the reflection, the lightstimulates fluid 440 to form acoustic waves 430 which travel towardtransducer 414. Transducer 414 can then detect acoustic waves 430 andoutput a signal through an electrical interconnect 434 to othercircuitry (not shown). Although transducer 414 is shown against asurface (416) that is outside of fluid 440, in other embodiments (notshown) transducer 414 can be mounted against a surface within fluid 440(such as, for example, surface 418).

Contact of block 400 with fluid sample 440 can be accomplished byinsertion of block 400 either entirely or partially into fluid sample440, and can comprise more than one surface in physical contact withfluid sample 440. Fluid sample 440 can comprise, for example, either aliquid or a gas.

FIG. 12 illustrates a sample cell 500 corresponding to an alternativeembodiment of the present invention. Sample cell 500 comprises convexcurved sidewall surfaces 502 and 504, a substantially planar frontsurface 506 configured to be proximate a sample, and a substantiallyplanar back surface 508 configured to be proximate a transducer 510.Curved surfaces 502 and 504 are preferably shaped as arcs of circles,.and are preferably substantially mirror images of one another. FIG. 12further illustrates a light beam 512 entering sample cell 500 throughsidewall surface 502, reflecting from surface 506, and exiting throughsidewall surface 502. As shown, curved sidewall surface 502 focuses beam512 so that beam 512 is narrowed upon passing through sidewall surface502. Curved sidewall surface 504 then defocuses beam 512 as beam 512exits sample cell 500. In the shown preferred embodiment, sidewallsurfaces 502 and 504 comprise curved regions extending an entirety of alength of the sidewall surfaces. It is to be understood, however, thatthe invention encompasses other embodiments (not shown) wherein thecurved regions of sidewall surfaces 502 and 504 extend along less thanan entirety of the length of sidewall surfaces 502 and 504.

An advantage of sample cell 500 over the above-discussed sample cellembodiments having planar sidewall surfaces, in addition to its focusingof a light beam, is that sample cell 500 can generate minimal amounts ofreflection with light beams entering sidewall 502 from a number ofangular directions relative to planar surface 506. In contrast, cellshaving planar sidewall surfaces, such as planar sidewall surface 114 ofcell 100 (FIG. 5), will generally reflect a substantial portion of alight beam unless the beam enters the sidewall surface at an anglenormal to the plane of the sidewall surface. Thus, cells having planarsidewall surfaces (such as cell 100 of FIG. 5) can generate minimalamounts of reflection with light beams entering the sidewall surfaces(such as surface 114 of FIG. 5) from only a very limited number ofangular directions relative to a front planar surface adjacent a sample(such as surface 110 of FIG. 5).

FIG. 13 illustrates a sample cell 600 corresponding to an yet anotheralternative embodiment of the present invention. Sample cell 600comprises concave curved sidewall surfaces 602 and 604, a substantiallyplanar front surface 606 configured to be proximate a sample, and asubstantially planar back surface 608 configured to be proximate atransducer 610. Curved surfaces 602 and 604 are preferably shaped asarcs of circles. FIG. 13 further illustrates a light beam 612 enteringsample cell 600 through sidewall surface 602, reflecting from surface606, and exiting through sidewall surface 602. As shown, curved sidewallsurface 602 defocuses beam 612 so that beam 612 is broadened uponpassing through sidewall surface 602. Curved sidewall surface 604 thenrefocuses beam 612 as beam 612 exits sample cell 600.

It is noted that in the photoacoustic sample cell embodiments describedabove, transducers are mounted to sample cell blocks through which alight beam is passed. It is to be understood, however, that theinvention encompasses other embodiments wherein transducers are mountedin other configurations such as, for example, to other surfaces incontact with a sample, or in acoustic contact with a sample without anintervening surface.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A photoacoustic spectroscopy apparatuscomprising; a transducer having a planar detecting surface; and meansfor directing a beam of light through a sample obliquely relative to theplane of the detector surface to generate acoustic signals detectable bythe transducer.
 2. A photoacoustic spectroscopy apparatus comprising: atransducer having a planar detecting surface; a wall of material havinga surface against a sample; and a light source oriented to direct lightthrough the wall of material and into the sample at an oblique anglerelative to the planar detecting surface to generate a plurality ofacoustic waves within the sample that are detectable by the transducer.3. The apparatus of claim 2 wherein the oblique angle is adjustable to afirst orientation and a second orientation, the light directed at thefirst orientation predominately reflecting from the surface against thesample, and the light directed at the second orientation predominatelypenetrating the surface against the sample and refracting into thesample.
 4. The apparatus of claim 2 wherein the oblique angle is greaterthan 20° and less than 70°.
 5. A photoacoustic spectroscopy methodcomprising: providing a transducer having a planar detecting surface;providing a wall of material having a surface against a sample;directing light through the wall of material and into the sample, thelight passing through the sample at an oblique angle relative to theplanar detecting surface; and generating a plurality of acoustic waveswithin the sample that are detectable by the transducer.
 6. The methodof claim 5 wherein the directing comprises one or more of refracting apredominate portion of the light from the surface and reflecting apredominate portion of the light from the surface.
 7. A photoacousticspectroscopy method comprising: providing a sample and anacoustically-stimulable transducer acoustically coupled with the sample,the acoustically-stimulable transducer comprising a detector surfacehaving a substantially planar portion; directing a first beam of lightthrough the sample at an angle oblique to the substantially planarportion of the detector surface to generate sound waves in the sample;and detecting the sound waves with the transducer.
 8. The method ofclaim 7 wherein the angle is greater than 10° and less than 80°.
 9. Themethod of claim 7 wherein the sample is against a first planar portionof a mass, and wherein the detector surface is along a second planarportion of the mass.
 10. The method of claim 9 wherein the first andsecond planar portions are opposing outer surfaces of the mass.
 11. Themethod of claim 7 wherein the sample is between two blocks, the samplebeing against a first planar portion of one of the blocks, and thedetector surface being along a second planar portion of said one of theblocks.
 12. The method of claim 7 wherein the angle is greater than 20°and less than 70°.
 13. The method of claim 7 further comprisingdirecting a second beam of light through the sample at an angle obliqueto the substantially planar portion of the detector surface to generatesound waves in the sample.
 14. The method of claim 13 wherein thedirecting the second beam of light occurs after the directing the firstbeam of light.
 15. The method of claim 13 wherein directing the firstbeam of light is in a first direction within the sample and thedirecting the second beam of light is in a second direction within thesample.
 16. The method of claim 15 wherein the first and seconddirections are substantially opposite to one another.
 17. The method ofclaim 15 wherein the directing the second beam of light occurs after thedirecting the first beam of light.
 18. A method of photoacousticspectroscopy, comprising: providing a sample; providing a block ofmaterial proximate the sample, the block having a substantially planarsurface adjacent the sample and comprising a critical angle relative tothe substantially planar surface; providing a transducer acousticallycoupled with the sample; directing a beam of light into the block at anangle oblique to the substantially planar surface, the angle beinggreater than the critical angle to generate an internal reflection ofthe light from the surface, the light generating sound waves in thesample during the reflection; and detecting the sound waves with thetransducer.
 19. The method of claim 18 wherein the beam of lightcomprises multiple wavelengths, and wherein only some of said multiplewavelengths are internally reflected from the surface.
 20. The method ofclaim 18 wherein proximate is inserting the block into the sample. 21.The method of claim 20 wherein the inserting into the sample is prior todirecting the beam.
 22. The method of claim 18 wherein the samplecomprises a fluid contained within a vessel.
 23. The method of claim 22wherein said fluid is a liquid.